The Seventh International Symposium on the Chemistry and Biological Chemistry of Vanadium

The 7th International Symposium on the Chemistry and Biological Chemistry of Vanadium - V7 Symposium- which will be held in Toyama, Japan, from the 6th to the 9th of October, 2010.
This Conference focuses on all aspects of Vanadium Chemistry and Biochemistry.


Scientific Topics
1) Vanadium Inorganic Chemistry - Coordination, Speciation and Structure
2) Vanadium Bioinorganic and Biological Chemistry
3) Vanadium Transport, Toxicology and Enzymology
4) Therapeutic Applications of Vanadium Compounds
5) Vanadium-Induced or –Catalyzed Reactions
6) New Materials Containing Vanadium and their Processes


Venue
V7 Symposium will take place at Toyama Shimin Plaza, Otemachi 6,
Toyama 930-0084, Japan.

Web page
http://www.vanadiumseven.com/

Functional models of vanadium haloperoxidases

In the living system enzymes are found that work as catalyst. The catalytic ability of an enzyme is found in the so called active site, which is located in a cavity or cleft in the enzyme. This active site differs among enzymes, not only in size but also due to the presence of different catalytically active amino acids. Because of this, enzymes can stabilize different transition states and catalyse different reactions. Enzymes are highly chemoselective, regioselective and sterioselective. In spite of extensive research activity in this area the exact role of vanadium compounds in biological systems is far from being fully understood. Because of the complexity inherent to biochemical processes, chemists often concentrate their attention on model systems. Studies on the interaction of vanadates with biologically important ligands in order to find structural and/or functional models of haloperoxidases are numerous and have been the subject of several reviews. For better understanding of working mechanism and role of metal, numerous vanadium complexes have been studied [91]. cis-Dioxovanadium(V) (VO2+) in acidic aqueous solution is first reported functional mimic of VBrPO [92]. This has been shown to catalyse the bromination of 1,3,5-trimethoxybenzene (TMB) as well as the bromide mediated disproportionation of H2O2. In a first step, (Scheme 1.1) H2O2 is coordinated to vanadium giving red oxoperoxo [VO(O2)+] and yellow oxodiperoxo [VO(O2)2–] complexes [93]. The ratio between these two species depends on the H2O2 concentration and the pH. In a second step these complexes combine, yielding dioxotriperoxodivanadium(V) [(VO)2(O2)3], which is considered to be the actual oxidant. cis-Dioxovanadium(V) functions at acidic condition (~ pH 2 or less), because at lower acid concentration the amount of monoperoxovanadate is insufficient for dimerisation to [(VO)2(O2)3]. When suitable nucliophile is not available, second equivalent of hydrogen peroxide reduces the brominium ion to bromide ion and singlet oxygen [94]. The formation of Br+ is rate determining and results in singlet oxygen and brominated products.

Oxidative bromination of 1,3,5-trimethoxybenzene (TMB) catalysed by [VO(OMe)(MeOH)(sal-oap)] using H2O2 as oxidant has been reported by Butler et al. [95]. The brominated product 2-bromo-1,3,5-trimethoxybenzene was obtained in good yield. The bromination of 1,3,5-trimethoxybenzene to some extent has also been catalysed by [VO(hybeb)]2− and [VO(bhybeb)]– (H2hybeb = 1,2-bis(2-hydroxybenzylamido)benzene, ) [96]. Oxidative bromination of salicylaldehyde to 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde catalysed by [VO2(sal-inh)]− (H2sal-inh = Schiff base derived from salicylaldehyde and isonicotinic acid hydrazide) encapsulated in the cavity of zeolite-Y has been reported by Maurya et al. [97]. The encapsulation of [VO(salen)] (H2salen = bis(salicylaldehyde) ethylenediimine) in zeolite Na-Y via the flexible ligand method is described. NaY-[VO(salen)] was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butylhydroperoxide (t-BuOOH) as the oxidant [98].

Other vanadium(V) complexes of multidentate ligands, e.g. salicylideneaminoacids, iminodiacetic acid, nitrilotriacetic acid, citric acid, tripodal ligands etc. are some common functional models for vanadate-bromo peroxidsase. Scheme 1.2 presents the widely accepted mechanism after carrying out many spectroscopic studies [99].

VANADIUM IN NATURALLY OCCURRING SYSTEM

Biological systems have developed haloperoxidases enzymes to catalyse the oxidation of chloride, bromide and iodide by hydrogen peroxide. On the basis of their cofactor requirement haloperoxidases are classified into the following three groups: heme-containing, vanadium-containing [56] and metal-free haloperoxidases [57]. Among them, vanadium haloperoxidase (VHPO) appears to be the most ubiquitous [58]. The bromoperoxidases involve in the polymerization of polyphenols holding the zygote to the membrane during the reproductive cycle of the cell [59]. The vanadium haloperoxidases represent a group of peroxidases that possess a single bound vanadate ion in a prosthetic group. These enzymes are able to oxidize a suitable electophilic halide species (X) to the corresponding hypohalous acid (HOX) in the presence of H2O2. The hypohalous acid species may further react to give a halogenated species. The presence of such enzyme in nature could do some way to explaining the formation of at least some of the wide diversity of halogenated compounds in environment [60].

The historical nomenclature convention of HPO is based on the most electronegative halide that the enzyme can oxidize (i.e., the chloroperoxidases (ClPO) can oxidize both Cl– and Br– and bromoperoxidases (BrPO) can oxidize Br– ). HPO does not have the driving force to oxidize the fluoride; however a fluorinating enzyme, fluorinase, has recently been isolated and is proposed to act by SN2 mechanism [61, 62]. In 1983, a naturally occurring vanadium-containing enzyme, vanadium bromoperoxidase (VBrPO), was discovered in the marine brown alga Ascophyllum nodosum [63]. It has trigonal bipyramidal coordination sphere including apical histdine and a meridionally bound oxo group; Figure 1.3 [64].
The oxidation state of the vanadium is +V and it does not change when hydrogen peroxide bind to give activated peroxointermidate species. Vanadium chloroperoxidases (VClPO), in the native and peroxo forms (Figure 1.4), have been isolated from the fungus Curvularia inaequalis [65, 66]. The enzyme contains α- helical with two four helix bundles as main structural motif, having 609 amino acids with molecular mass of 67488 Da. The active site is located at the top of the bundles [67]. A five-coordinated trigonal bipyrimidal V(V) moiety (Figure 1.4) is present in the native form which is coordinated by three nonprotein oxo groups in the equatorial plane and one His496 and a hydroxy group at the axial positions. The oxygens are hydrogen bonded to several amino acid residues of the protein chain. The nitrogen of His496 coordinates to vanadium and hence is the only direct bond from the protein to the metal center. The apical V-O bond length is 1.93 Å and the V-N bond is 1.96 Å, three equatorial V-O bonds are about 1.65 Å long.
In the peroxo form (Figure 1.5), the peroxide ligand is bound in a η2-manner in the equatorial plane. The apical oxygen ligand detaches and gives a distorted tetragonal pyramid coordination geometry around the vanadium centre with the two peroxo oxygens having V-O bond length ~ 1.87 Å and O-O is 1.47 Å. One oxygen (V-O bond length 1.93 Å) and the nitrogen (V-N bond length 2.19 Å) are in the basal plane while one oxygen (V-O bond length 1.60 Å) is in the apical position.
Carallina officinalis has been isolated from red algae and shows a high degree of amino acid homology in their active centre and has almost identical structural feature as have been reported for other enzymes [68].

In the last decade peroxidases, particularly chloroperoxidase have been shown to catalyse a variety of synthetically useful oxygen transfer reactions with H2O2, including enantioselective oxidation of sulfides [69, 70]. Vanadium haloperoxidases, such as vanadium chloroperoxidase from Curvularia inaequalis are much more stable. The vanadium-dependent bromoperoxidase from Corallina officinalis mediates the enantioselective oxidation of aromatic sulfides [71, 72]. The brown seaweed Ascophyllum nodosum mediate the formation of the (R)-enantiomer of the methyl phenyl sulfoxide with 91 % enantiomeric excess, whereas the red seaweed Corallina pilulifera mediates formation of the (S)-enantiomer (55 % enantiomeric excess) under optimal reaction conditions [73]. Recently Butler et al. have reported vanadium bromoperoxidase catalyzed biosynthesis of halogenated marine natural products [74].
In 1986, a vanadium nitrogenase enzyme was isolated from each of the two soil bacteria, Azotobacter chroococcum [75] and A. vinelandii [76, 77]. In vanadium-nitrogenases, vanadium is in a low to medium oxidation state as an integral part of an iron-sulphur cluster [78] which activates and reductively protonates various unsaturated substrates [79]. The coordination sphere around vanadium is probably octahedral (33 in Figure 1.6) and is similar to that of molybdenum in the structurally characterized molybdenum-nitrogenase [78, 80]. Thus, in vanadium-nitrogenase, vanadium is coordinated to histidine and the vicinal hydroxide and carboxylate groups of homocitrate in addition to three sulphide ions from iron-cluster.
The vanadium-iron protein of vanadium nitrogenases extracted from Azotobactor chroococcum contains an iron-vanadium cofactor. It is suggested that the nitrogen fixation requires specific interactions between Fe-V cofactor (Fe-Vaco) or Fe-Mo cofactor (Fe-Moco) and their respective polypeptides [81]. The vanadium nitrogenases [82] are multicomponent metalloenzyme complexes that are capable of reducing dinitrogen to ammonia, and hence to a form accessible by plants [83].
Many nitrogenases consist of a Fe-S cluster and a molybdenum-dependent component [84]. The first vanadium containing nitrogenase, i.e. a V-Fe-protein, was isolated and purified in 1986 from certain nitrogen-fixing bacteria [75].
Complexes have been designed that model parts of the structure of the V-Fe cofactor. The coordination atom is S in place of N and/or O coordination. Dixovanadium(V) complex (34 – 39 in Figure 1.7) having O,N, and S atoms have been prepared and structurally characterized in order to model in order to model nitrogenases [85 – 90].

Antiamoebic activity of Vanadium

Vanadium complexes have also shown encouraging results on the in vitro antiamoebic activity. Amoebiasis is the infection of the human gastrointestinal tract by Entamoeba histolytica, a protozoan parasite that is capable of invading the intestinal mucosa and can infect almost every organ of the body. The most frequent form of extra intestinal amoebiasis is the amoebic liver abscess. Being responsible for approximately 100,000 deaths annually, placing it a second only to malaria in mortality due to protozoan parasite [50, 51]. Metronidazole (MNZ, 1-(2-hydroxyethyl)-2-methyl-5-nitroimidazole) is the most effective antiamoebic medication [52]. This, however, induces certain tumors in rodents and is mutagenic towards bacteria [53].

The ideal treatment for amoebiasis does not yet exist, mainly due to the toxicity of current antiamoebic drugs [54]. In vitro tests of the antiamoebic activity of dioxovanadium(V) complexes [K(H2O)n[VO2(X-sal-sbdt)] ( n = 2 or 3, H2X-sal-sbdt = Schiff base derived from salicylaldehyde and S-benzyldithiocarbazate, X = H, 5-Cl, 5-Br) against Entamoeba histolytica, show comparable (when X = H and 5-Cl) or substantially better (when X = 5-Br) amoebocidal action than metronidazole [55].

Antitumor activity of Vanadium

Various vanadium complexes are effective for antitumor activity; bis(cyclopentadienyl)-cis-dichlorovanadium(IV) and peroxovanadated(V) are the most active [48, 49]. Dioxovanadium(V) complexes of the type [VO2(X-sal-tsc)] [where H2X-sal-tsc = Schiff base derived from salicylaldehyde, 5-bromosalicylaldehyde, 2-hydroxy-1-naphthaldehyde and semicarbazide, N4-n-butylsemicarbazide and N4-(2-naphthyl)semicarbazide] exhibit selective cytotoxicity on TK-10 human tumor cell lines. A significant effect on the antitumor activity of the vanadium complexes by structural modifications on the semicarbazone moiety has been indicated [50]. Peroxovanadates(V) with or without organic ligands also exhibit antitumor activity. The active roles of hetero ligands in modifying the antitumor and toxicity of peroxovanadium(V) have also been indicated. A the set of 14 oxoperovanadium(V) complexes of the types (NH4)4[O{VO(O2)2}2], M3[VO(O2)2(C2O4)] and M[VO(O2)L] (L = malate, citrate, iminodiacetate, nitrilotriacetate and ethylenedianinetetraacetate) has been tested for the toxicity and antitumor activity against L1210 murine leukemia to examine the biological properties. Study shows that these complexes increase the life span by about 25 % [51].

Insulin mimetic property of vanadium

Vanadium was widely used as a therapeutic agent in the late 18th century, treating a variety of disease including anemia, tuberculosis, rheumatism and diabetes [18]. Vanadium may play significant role for nutrition in human [6]. Several nutritional studies have revealed that the deficiency of vanadium may impediment the proper growth and development of chick and rat [19].
Diabetes mellitus is one of the most threatening and costly epidemics [20] and classified as Type I, insulin dependent or Type II, non insulin dependent (Type I: where there is no production of insulin in the pancreatic β–cells and thus patients must take exogenous insulin to supplement its deficiency and Type II: where there is insulin production in the body, but its secretion or cellular response is not satisfactory). People with Type II diabetes are not dependent on exogenous insulin as much as patients with Type I diabetes, but may need it to control their blood glucose levels [21]. Non- insulin dependent diabetes mellitus is most common form of diabetes in adult humans [22]. Insulin is a hormone which is essential both for metabolising of fat and carbohydrates [23]. The increased insulin promotes glucose uptake by the liver and gut, as well as by peripheral tissue (adipose and muscle), which results in the energy production and storage as needed by the organism [24]. Insulin is not orally active. As such oral ingestion of exogenous insulin does not work as biologically active hormone.
Obesity is an important risk factor for the development of insulin resistance in Type II diabetes [25]. The majority of patients with Type II diabetes are obese, and it has been demonstrated that weight gain correlates with deterioration of insulin resistance, whereas weight loss correlates with the improvement of insulin sensitivity.
A new turning point occurred in 1985, when Heyliger et al. demonstrated that oral administration of vanadate to streptozocin- treated diabetic rats (STZ rats), lowered the high level of glucose to normal, though they have a high toxicity. During the last decade, vanadium compounds have been found to act like insulin in all three main target tissues of the hormone, namely skeletal muscles, adipose and liver. Vanadium compounds are, therefore, of particular interest. By contrast vanadium compounds can be orally administrated, thereby potentially eliminating the need for daily insulin injection in diabetic individuals [26]. Since then research has been undertaken to find insulin-mimetic vanadium compounds to be used as oral substitute of insulin [27]. Vanadium ions show in vitro insulin-mimetic effect [28]. Low molecular weight metal complexes enhance the lipophilicity, membrane transport and bioavailability. The vanadyl V(IV) is less toxic to rats than the vanadate V(V) state [29] and hence vanadium(IV) state is proposed to be a possible active form of vanadium in mimicking or enhancing insulin action by interacting with the glucose transporter. The oral treatment with vanadate improves insulin sensitivity in skeletal muscle of Type II diabetic patients and results in reduced fasting plasma glucose concentration and suppression of hepatic glucose production [30].
Insulin activities of vanadium compounds are related to their potent inhibition of protein tyrosine phosphatases (PTPs). The organovanadium compounds have been shown to have superior insulin activities probably as a consequence of better bioavailability of these compounds or more potent activity at enzyme active site. Bis(maltolato)oxovanadium(IV) (BMOV), a potent insulin sensitiser, was shown to be a reversible, competitive phosphatase inhibitor that inhibited phosphatase activity in cultured cell and enhanced insulin receptor activation in vivo [31]. In fact, bis(maltolato)oxovanadium(IV) (BMOV) is the most widely tested complex among many proposed insulin mimetic vanadium complexes [27, 32 – 35]. The efficacy of the drug is possibly due to its interaction with human serum albumin (HSA) [36]. The generation of VO4 from BMOV in ‘physiologic’ solutions and the uncomplexed vanadium as an active component has been suggested by Peters et al. [31]. The closely related analogue bis(ethylmaltolato)oxovanadium(IV) (BEOV) has completed phase I clinical trials for the treatment of Type II diabetes mellitus and study suggests that there were no adverse health effects in any of the (nondiabetic) volunteers [37]. Oxidation state of metal ion, interaction of complexes with human serum albumin (HSA) [36, 38] and design of ligands have been indicated to play an important role in modifying the biological effects of metal based drugs [39].
Several types of neutral and low molecular weight vanadium(IV) complexes with organic ligands have been designed and investigated in animal model systems for the treatment of diabetes. Vanadium-dithiocarbamate complexes have been reported as potent orally active insulin-mimetic for the treatment of insulin-dependent diabetes mellitus in rats. Sakurai et al. [40], and Rehder et al. [41] have screened toxicity and insulin mimetic activity of a whole range of oxovanadium(IV), oxovanadium(V) and oxoperoxovanadium(V) complexes 1 – 17 presented in Figure 1.1; while details of other complexes (18 – 32 in Figure 1.2) screened for insulin-mimetic activity have been provided in review articles [23, 42]. Ammonium salt of dipicolinato oxovanadium(V) is a clinically useful oral hypoglycemic agent with no toxicity in cats with naturally occurring diabetes mellitus [42]. Simple peroxo compounds have also been screened for their insulin-mimetic action [43, 44].
It has been observed that the mixture of H2O2 and vanadate or vanadium(V) oxide were more potent in controlling the blood glucose level in rats than either vanadate or H2O2 alone [45, 46]. Literature also cites insulin-mimetic properties of peroxovanadium(V) compounds with nucleic bases such as uracil and cytocin [47].

TOXIC EFFECT OF VANADIUM

Vanadium is toxic both as a cation and as an anion [15]. The toxicity of vanadium has been found to be high when it is given intravenous, low when it is orally administered, and intermediate in the case of respiratory exposure. Toxicity as observed by weight lose, poor appetite, vomiting and diarrhea has been associated with ingestion of vanadium compounds therefore the therapeutic index of some vanadium complexes can be quit narrow [16]. Recent efforts focused on identifying vanadium compounds with increased therapeutic potency and decreased toxicity. The recent successes achieved on transition metal complexes having organic ligands suggest that modification of the metal ion on combining with organic ligands not only increases efficacy but also decreases toxicity [17]. V(III) is readily oxidized to V(IV) and V(V) at physiological pH and V(IV) is lees toxic than V(V).

VANADIUM IN BIOLOGY

Many essential/ trace elements play an important role in a number of biological processes by activating or inhibiting enzymatic reactions, by competing the permeability of cell membranes, maintaining genomic stability and/or by other mechanisms [5].
The role of vanadium in biochemistry has attracted attention for the last three decades. It could be used as inhibitor for nucleases and phosphatases [6]. Cantley and coworkers revealed a potent inhibitor of Na+/K+ – ATPase, widely used to study the mechanism of the sodium-potassium pump [7,8]. This pump is necessary for proper transport of materials across cell membranes to maintain ionic equilibrium.
Vanadium(V) (H2VO4−) enters into cells probably through the phosphate transport mechanism and reduced to vanadium(IV) through one-electron reduction in the gastrointestinal tract. VO2+ undergoes auto oxidation to vanadate in the presence of oxygen, whereas glutathione, ascorbate, cysteine and similar reducing agents can reduce vanadate. Thus, endogenous reducing agents and dissolved oxygen ensure that both vanadium(V) and vanadium(IV) species are present in serum. Vanadium has been found in its +IV oxidation state in mammalian lung and heart tissues [9] whereas +V oxidation state is common in kidney, liver and erythrocytes. It has also been suggested that extra cellular vanadium exists primarily as vanadium(V) and exclusively vanadium(IV) inside the cells [10]. The processes of interactions between vanadium species and serum albumin are still a challenge for scientists. However, the deposition of the vanadium in different tissue, obtained from in vitro experiments performed in different laboratories on several animals, follows the order: bone > kidney > liver > spleen > intestine  stomach, blood, muscle, testes, lungs and brain. The excretion of the small fraction of ingested and not retained vanadium occurs mainly through urine, as low molecular weight VO2+ complexes. Biliar excretion seems to be a secondary route [11–14].

INTRODUCTORY

Vanadium, named after the Nordic Goddess “Vanadis”, is a trace element that occurs in concentrations ranging from 0.1 – 3.0 nmol/g in most mammalian cells. Its concentration is about 136 ppm in the earth’s crust and is nineteenth element in the order of abundance. Anthropogenic sources include the combustion of fossil fuels, particularly residual fuel oils, which make up the single largest overall release of vanadium to the atmosphere. Vanadium has been reported to be an essential bio-element [1] for certain organisms, including tunicates, bacteria and some fungi. The physiological role of vanadium is not known but its importance has been indicated for the normal growth and development [2]. Crans and coworkers have outlined five specific ways by which vanadium interacts with proteins [3, 4]. Vanadium with atomic number 23 and electronic configuration [Ar]3d34s2, can exits in at least six oxidation states. Oxidation states +III, +IV and +V are most common but +III oxidation state is reducing in nature and difficult to exist in aqueous (pH~ 7.0) solution. Oxidation states +IV and +V are generally stabilized through V-O bond, and oxocations [VO]2+, [VO]3+ and [VO2]+ are most common for biological systems.
The presence of vanadium in vanadium based enzymes e.g. vanadate-dependent haloperoxidases and vanadium nitrogenase attracted attention of researchers to develop coordination chemistry of vanadium in search of good models for these enzymes. Studies on the metabolism and detoxification of vanadium compounds under physiological conditions, stability and speciation of vanadium complexes in biofluides, and potential therapeutic and catalytic applications have further influenced the coordination chemistry of vanadium.

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